Dee20023 - Chapter 1
Dee20023 - Chapter 1
Dee20023 - Chapter 1
SEMICONDUCTOR DEVICES
CHAPTER 1
SEMICONDUCTOR
ATOMIC STRUCTURE
All matters on earth made of atoms (made up of elements or
combination of elements).
All atoms consist of electrons, protons, and neutrons.
An atom is the smallest particle of an element that retains the
characteristics of that element.
According to Bohr, atoms have a planetary structure that consists of a
central nucleus, surrounded by orbiting electrons.
Nucleus contains protons and neutrons.
Bohr’s Atomic Structure
The ability of a material to conduct current is based
on its atomic structure.
The orbit paths of the electrons surrounding the nucleus
are called shells.
Each shell has a defined number of electrons it will hold.(2xn2)
Ex:Copper:2.8.18.1 (electron valence)
The outer shell is called the valence shell and electrons at this layer are
called valence electrons.
• The orbit paths of the electrons surrounding the
nucleus are called shells.
• Each shell has a defined number of electrons it will
hold.(2xn2)
Ex:Copper:2.8.18.1 (electron valence)
• The outer shell is called the valence shell and
electrons at this layer are called valence electrons.
• Maximum number of valence electron is 8
Contains 5 to 8 e.v
Contains 1 to 3 e.v
Contains 4 e.v
INSULATOR :
An atom of a material carries 5 to 8 valence electron
High resistance, could not conduct current
The atom has more tendency to gain free electrons to complete its
shell. ( Argon, Neon)
SEMICONDUCTOR :
An atom of a material carries 4 valence electrons,
A material that has a conductivity level somewhere between a
conductor and an insulator.
It is not easy for the atom to lose or gain any electrons. (Silicon,
Germanium, Carbon)
13
CONDUCTOR
3 ELECTROVALENCE
16
INSULATOR
6 ELECTROVALENCE
14
SEMICONDUCTOR
4 ELECTROVALENCE
COVALENT BOND
In a pure silicon or germanium crystal, the four valence electrons of
one atom form a bonding arrangement with four adjoining atoms.
This bonding of atoms, strengthened by the sharing of electrons, is
called covalent bonding – a method by which atoms complete their
valence shell by sharing valence electrons with other atoms
Intrinsic (pure) materials are those semiconductors that have been carefully
refined to reduce the impurities to a very low level.
Intrinsic (pure) Si and Ge are poor conductor due partially to the number of
valence electrons, covalent bonding and relatively large energy gap.
Extrinsic materials are those semiconductors that have been subjected to
doping process and no longer pure.
Doping is the process of adding impurity atoms to intrinsic Si or Ge to
improve the conductivity of the semiconductor.
EXTRINSIC SEMICONDUCTOR
There are two types of semiconductor material that are subjected to doping
process which are :
a) N-type Extrinsic
b) P-type Extrinsic
Two types of elements used for doping are :
a) Pentavalent – element that has five valence electrons
b) Trivalent – element that has three valence electrons
N type
Both materials are half conductors because they have four valence electrons.
The nature of the material is that it is not easy to release electrons as easy as
conductors and it is also not easy to receive electrons as easy as insulation.
What is the Covalent Bond? Draw a diagram showing the
Covalent Bond for Germanium.
The Covalent bond is the process of electron valence sharing of an atom with the valence
electrons of its neighboring atom to make the one of the atoms stable..
State 4 factors that allow electrons to be free from Covalent
bond.
i. Heat
ii. Potential Difference
ii. Temperature Increment
iv. Doping / Absorption Process
What is the impulse? Explain each one
Examples :
Type-N Material
Majority Carrier : Free Electrons (negative charge)
Minority Carrier : Holes
Type-P Material
Majority Carrier: Holes (positive charge)
Minority Carrier : Free Electron
Review Questions :
1. What is a semiconductor?
11. The doping process of antimony into silicon materials will result an a/
an __________ material.
The n‐type region has many free electrons (majority carriers) and only a few thermally
generated holes.
The p‐type region has many holes (majority carriers) and only a few thermally generated
free electrons (minority carriers).
With the formation of the p and n materials, holes from p ‐type will diffuse into the n‐type,
and electrons from n‐type will diffuse into the p‐type. This process is called recombination.
N-type P-type
Movement of electron-hole
The p‐region loses holes,
and become negatively
charged
The p-type material is connected to the positive terminal and the n-type is
connected to the negative terminal.
• Positive terminal of the battery will push the holes in the p-region towards the
junction.
• Hence, the recombination process occurs and the number of negative ion in the p-
region near the junction decreases.
• Negative terminal of the battery will push the free electron in the n-region towards
the junction and recombine with the positive ion.
• Hence, the number of positive ion decreases. As a result, the number of uncovered
ions is reduced and the width of the depletion region will also reduce.
• Since, the barrier potential is now reduced; electrons in n-type will be easily moved to
the p-region.
Reverse Bias condition (VD< 0V)
The p-type material is connected to the negative terminal and the n-type is
connected to the positive terminal.
• The positive terminal of the battery extracts free electrons from the n-
region and the negative terminal extracts free holes (positive ions) from
the p-region.
• The number of uncovered positive ion in the n-region and negative ions
in the p-region will increase. Hence, the width of the depletion region
increases.
• However, in real P-N junction, there is a small flow of current in the
reversed-biased mode.
• As the temperature is high enough, there will be a current exists is called
the reverse-saturation current, Is or leakage current
Breakdown Voltage
When a reverse-bias is applied to a p-n junction (diode), the electric field in
the depletion region increases.
The electric field may become large enough that covalent bonds are
broken and electron hole pairs are created.
Electrons are swept into the n-region and holes are swept into the p-region
by the electric field, generating a large reverse current. This phenomenon
is called breakdown.
This current can be limited by the external circuit. If this current is not
limited, a large Power can be dissipated in the junction that may damage
the device and cause burnout.
Review
1. Draw and state the meaning of forward biased voltage and
reverse biased voltage supplied across a P-N junction